1. Field of the Invention
The invention relates to scanning cathode ray tubes, and more particularly to cathode ray tubes incorporating a folded electron beam path en route to the display screen.
2. Description of Related Art
A typical cathode ray tube comprises an evacuated glass envelope having a neck portion oriented orthogonally to a fluorescent display screen. The neck region contains one or more electron guns, and opens up into a funnel or cone section which terminates in the display screen. A biaxially controllable magnetic deflection yoke is disposed coaxially outside the tube at the junction between the neck section and the cone section.
It is known that electron beam quality can be improved by the use of certain electron guns that either are longer than the electron guns in conventional use today, or have a larger diameter than conventional electron guns. Longer guns are not used in part because they would add depth to the television display cabinet, which is undesirable from a commercial point of view. Larger diameter guns are not used presumably because they would require a larger diameter neck, which in turn would require (in most cases) a larger diameter yoke. A larger diameter yoke would in turn require larger drive currents or greater interaction lengths in order to achieve the same horizontal and vertical deflection angles.
One way to accommodate a longer or larger diameter electron gun might be to create a cathode ray tube having a folded electron beam path en route to the display screen. Some examples of such tubes are illustrated in U.S. Pat. Nos. 2,164,555, 2,464,562, 2,728,025, 2,945,974 and 3,412,282, all incorporated by reference herein. In each of these tubes, the electron beam is subjected to both horizontal and vertical deflection, with a subsequent folding of the beam path en route to the screen. However, it is believed that the systems of these patents are suitable only for small size tubes, due to the limitations of the two-dimensional beam folding mechanisms that are used. Such limitations include both beam absorption and image distortion.
In Schwartz U.S. Pat. No. 4,739,218, a cathode ray tube is disclosed in which the horizontal and vertical deflection mechanisms are separated, and the electron beam path is bent by 90xc2x0 in the region between the separated deflection mechanisms. The Schwartz system avoids many of the problems of the prior art, but still has some difficulties of its own. For example, in an embodiment in which the right angle bender operates electrostatically, a number of additional components are required inside the glass. Such components are not always compatible with existing CRT production lines. In addition, significant deflection defocusing can sometimes occur due to the differential energy change that can occur at the opposite edges of the electron beam.
The Schwartz patent also indicates that the right angle bender mechanism may be magnetic instead of electrostatic. A magnetic bender and deflection structure would simplify the internal structure of the tube because all of the magnetic components can be placed outside the tube. However, as indicated in the patent, the stability of the bending angle produced by the right angle magnetic bender depends on the stability of the high voltage accelerating potential applied to the tube. That is, whenever the high voltage potential increases or decreases slightly, the velocity of the electron beam entering the bender will also increase or decrease. This will result in an uncontrolled reduction or increase in the bending angle. Instability in the high voltage supply therefore appears as undesirable instability in the vertical position of the image produced on the display.
Another difficulty with a magnetic right angle bender is that the magnetic field in some embodiments can extend or xe2x80x9cleakxe2x80x9d back into the gun area, causing the beam to enter the bender off-center. It may be possible to design and install a xe2x80x9cpusherxe2x80x9d magnet along the electron beam path between the gun and the bender, to counteract the effect of the bender leakage magnetic field in that region by pushing the partially deflected electron beam back into the center of the desired beam path, but this arrangement can greatly compound the sensitivity of the bending angle to changes in the high voltage supply.
In addition to the above difficulties, a magnetic right angle bender also does not completely avoid deflection defocusing because in some embodiments, diametrically opposite edges of the electron beam will be affected differently by the magnetic bender field. In particular, the outer edge of the beam as it makes the bend has a greater interaction length with the magnetic field, and therefore is bent by a slightly greater angle than is the inner edge of the beam. Thus an edge crossover occurs either as the beam passes through the bender or at some point downstream of the bender. One might suspect that a system can be designed to position the edge crossover point all the way out at the display screen and thereby improve focus at the display screen, but it turns out that the edge crossover point is very difficult to control and in fact renders focus at the display screen problematical.
Accordingly, there is a need for a folded electron beam path cathode ray tube which overcomes the problems of the electrostatic and magnetic right angle benders of the prior art.
According to the invention, roughly described, a cathode ray tube contains an electron beam source whose central axis is essentially parallel with the plane of the tube""s fluorescent display screen. The path of the beam undergoes large angular bending through a magnetic loop bender, causing the beam to intersect itself before exiting the bender orthogonally with the screen. Because of the symmetries inherent in a loop bender, the bending angle is not sensitive to variations in the beam velocity or the high voltage potential of the tube. Downstream of the bender, the beam is deflected by conventional biaxial means before impingement on the screen. A magnetic field stop can be added at the entrance port of the bender in order to prevent the bender""s leakage field from affecting the beam path before it enters the bender""s main field. A magnetic field stop can also be added at the exiting port of the bender to isolate the bender magnetic field from the yoke magnetic field. Preferably both magnetic field stopping functions are performed by a single magnetic flux conductor having a surface nearest the bender magnet structure, which surface lies in a plane that passes through the beam intersection point (i.e. that point where the beam entering the bender intersects the beam leaving the bender) and that lies parallel to the pole termination plane of the bender magnetic structure.
The overall result of this configuration is a large reduction in the physical depth of the tube, as well as the provision of sufficient space for a large electron gun for the purpose of minimizing beam aberration. A longer electron gun can be used because it does not increase the overall tube depth, and a larger diameter electron gun can be used because the diameter of the neck section of the tube imposes no constraints on the diameter of the section around which the yoke is disposed.
An astigmatic beam shaping mechanism can also be included prior to the conventional deflection yoke. In addition, post-deflection acceleration of the beam can provide further shortening of the tube depth as well as a focusing action that reduces the effect of beam enlargement due to mutual electron repulsion within the beam. All of the beam bending and deflection can be done by externally attached components so that there are no internal electrostatic elements other than those in the electron gun and the conventional conductive inner coatings. For color applications, the well-known beam index method may be used. The overall display size of a tube made according to the invention is limited only by the size of the glass enclosure, and production may be carried out on conventional established CRT production lines.